1. Field of the Invention
The present invention relates to a cleaning method employed for removing contaminants adhering to surfaces of substrates and the like during processes for manufacturing liquid crystal display substrates or semiconductors, and also relates to an apparatus for such method.
2. Description of the Related Art
A process for manufacturing a liquid crystal display substrate includes: a step for forming a metallic film (conductive film) by sputtering a metal such as ITO to the substrate surface; a step for forming electrodes by partially removing the metallic film (conductive film) by etching after forming a resist layer on the metallic film; and a step for forming an orientation film on the electrode made of the metallic film (conductive film). A process for manufacturing a liquid crystal display substrate employing TFT includes: a step for forming a metallic film (conductive films) by sputtering or CVD to form TFT; a step for coating a resist layer; and a step for partially removing the metallic film by etching.
During the above manufacturing steps, particles in the air may adhere to the substrate surface, metal or organic substances may also adhere during etching or resist-layer forming, and an oxide film may be spontaneously formed over the substrate surface. If these contaminants adhere to substrates, electrodes, or the interfaces between electrodes and the orientation film, contact between electrodes deteriorates, thereby increasing resistance and also causing inferior wiring. Therefore, steps for removing these contaminants are significantly important in order to produce high performance elements, and the substrate surface must be cleaned at every manufacturing step. In particular, reliable cleaning is necessary for a TFT forming step in which each metallic film must be layered on a highly clean interface.
An aqueous acidic cleaning solution containing, e.g., HCl or H.sub.2 SO.sub.4, or an aqueous alkaline cleaning solution containing, e.g., NH.sub.4 OH, are employed for conventional cleaning steps. Particles are washed away with an acidic solution and metal and organic substances are washed away with an alkaline solution. Oxide films can be removed by hydrofluoric acid.
Although cleaning power is strengthened by increasing the concentration of the alkaline or acid solution, an exceedingly high concentration causes damage on the substrate surface or corrodes electrodes formed on the surface. Moreover, the type of contaminants adhering to a substrate during each manufacturing step differs from step to step, and one type of aqueous cleaning solution can remove only one type of contaminant. Therefore a separate cleaning procedure using an exclusive cleaning solution is necessary for each step. For example, in a cleaning step after electrode forming, at least two types of solutions, i.e., an acidic solution and an alkaline solution, must be separately applied to a substrate surface to remove metal and particles adhering to the surface. Thus, an inefficiently longer time is required for the cleaning step. In addition, since each cleaning step is conducted in a clean room to prevent contaminants in the air from adhering to substrates, a separate cleaning chamber must be prepared for each aqueous cleaning solution. Consequently, when a plurality of aqueous cleaning solutions are used corresponding to a plurality of cleaning procedures, a corresponding number of cleaning chambers are required, thereby increasing the size of cleaning apparatuses and equipment cost.
To solve the above problem, super-pure water alone or in combination with ultrasonic-waves is employed for the cleaning step in TFT-LCD substrate manufacturing, instead of the above acidic or alkaline solution. Super-pure water is highly purified such that micro-particles, colloidal microorganisms, organic substances, metal, ions, dissolved oxygen and the like are removed to extremely low concentrations by membrane equipment such as ultrafiltration membrane and reverse osmosis membrane, ion-exchange equipment, ultraviolet irradiation equipment, and the like. By the use of super-pure water, substrates or electrodes will not corrode during the cleaning step, and furthermore, no contaminants are derived from super-pure water.
However, super-pure water has low cleaning power and is not efficient in the removal of contaminants. In addition, super-pure water cannot removed certain types of contaminants from the substrate surface. Therefore, to clean a substrate using super-pure water, for example, the following procedures are required: the substrate surface is exposed to ultraviolet irradiation in the presence of ozone to remove organic substances, brushed for removing large particles, washed with super-pure water in combination with ultrasonic-waves to remove small particles, and then washed with super-pure water.
Since the cleaning step is divided into several procedures, as is above described, a plurality of cleaning chambers must be prepared. Moreover, since the cleaning step is carried out for a long period of time while using a large quantity of super-pure water, lower cost of manufacturing substrates cannot be achieved.
In addition, the substrate is transferred from chamber to chamber according to the above cleaning step, therefore contaminants may re-adhere to the substrate during the transfer procedure. Furthermore, since metal cannot be removed by washing with super-pure water, adhesion of metallic impurities is unavoidable, resulting in insufficient cleaning.